Multi-wavelength lasers (MWLs) are useful in providing multiple-channel light sources for optical networks, especially for dense wavelength division multiplexing (DWDM) systems. The desirable features of MWLs used for DWDM systems include compact design, high stability in the output wavelengths and wavelength spacing, reasonable output power, wide wavelength range and narrow linewidth for a large channel capacity, convenience of modulating individual wavelength at a high rate, and easiness of manufacture. In addition, the wavelength locking mechanisms of laser sources should be as simple as possible. No existing MWL provides all those features largely because of the limitations in current laser technologies.
A number of MWL technologies have been investigated, and they can be generally classified into two types: MWL array and shared-gain MWLs. An MWL array consists of a row of single-wavelength lasers such as distributed feedback (DFB) lasers and distributed Bragg reflector (DBR) lasers. Each DFB or DBR laser in the laser array can be tuned and locked in a channel, and modulated individually.
One of unsolved problems with DFB or DBR MWLs is uneven wavelength drifting that can cause cross talks. The output wavelength of a channel depends upon the combined effects of junction temperature and injection currents (diode pumping current, wavelength tuning current, and phase shifting current). The channel tuning mechanisms of current MWL arrays are very complex. Since the output wavelength of an individual laser in the array is a function of its junction temperature, any fluctuation in the temperature of the diode will cause its output wavelength to drift. To keep output wavelengths locked in their channels, it is essential to maintain the junction temperature constant. Heat sinks capable of precisely controlling temperature are ordinarily used to keep output wavelengths stable. Also, the costs for making such laser arrays are very high.
In shared-gain MWLs, laser channels share one gain region by integrating multi-resonance feedback elements with one gain medium, thereby yielding a number of output wavelengths. One of the advantages of this design is stable output wavelength spacing even when all output wavelengths drift simultaneously. It is more difficult to modulate individual channels for most of the existing shared-gain MWLs than for MWL arrays. U.S. Pat. No. 6,289,032 discloses a self-collimating MWL laser, which simultaneously permits broad-beam collimation and monomode operation, with simultaneous emission of multiple wavelengths from a single aperture. While this design results in stable wavelength spacing, it does not allow direct or internal modulation of each individual wavelength.